Magnetization transfer (MT) imaging (MTI) provides a method to investigate the properties of protons that are bound to macromolecules in tissue. MTI has been used in clinical applications to investigate pathology-induced changes in tissue characteristics, such as multiple sclerosis (MS) white matter lesions, dementia, traumatic brain injury, and tumors, or to augment the contrast or visibility of low-MT tissue, such as blood in MR angiography. Recently, MTI has been shown as a promising technique to characterize tissue fibrosis.
In MTI, off-resonance RF pulses (e.g., a widely applied Gaussian RF pulse) are dominantly used with a bandwidth of several hundred hertz (Hz) and typically applied at a frequency offset at least 1 kHz from the mobile water peak to minimize direct saturation. To achieve off-resonance saturations of bound water spins within a large frequency range, the bandwidth of the MT RF pulses has to be increased. A consequence of the increased bandwidth is an undesired augmentation of direct saturation, unless the MT pulses are shifted further away from the mobile water. However, shifting the MT pulses further off-resonance has several disadvantages, including a decreased ability to saturate bound spins close to water and reduced MT contrast.
Furthermore, in a traditional MTI, the MT pulses are typically only applied with an offset frequency on one side of the mobile water peak (i.e., asymmetrical). It is well known that MT contrast in tissue can be asymmetric. To achieve a complete assessment of the pathology-induced MT effects from bound water spins with off-resonance frequencies both lower and higher than the mobile water peak, two MTI scans have to be performed, each applying an MT pulse with an offset on either side of the mobile water frequency, or the MT preparation has to be performed two times with two opposite frequency offsets before the imaging readout. Although the latter approach is more efficient than the first one, it prolongs time response and increases preparation waiting time and imaging readout time, with its MT contrast less optimal than that using the multi-banded RF pulses.